Accurate polarization characterization of thin film polarizing beamsplitters with cavity ring-down technique

The cavity ring-down (CRD) technique is employed to accurately measure the polarization characteristics of thin film polarizing beamsplitters (PBS's). The measurement principle and related theory are provided. By establishing s- and p-polarized two-channel CRD configurations, the s- and p-polarization transmittance and reflectance values (T_s, T_p, R_s, and R_p) and the polarization extinction ratio (PER = T_p/T_s) of the thin film PBS's are measured with high accuracy. The experimental results with a 45-degree and a Brewster angle PBS demonstrate that the two-channel CRD is capable of measuring the p-polarization transmittance T_p of ∼99.0% and higher with an error of ±0.01∼0.02% or lower, and the s-polarization transmittance T_s < 1 ppm with an uncertainty of < ±0.01 ppm or ∼ ± 1.0%. A PER as high as 10⁶:1 is determined by CRD with an error of only ±1.0∼2.0%. Comparisons between PER measurements using CRD and spectrophotometry show that that CRD offers much higher accuracy for high PER measurements than spectrophotometry. The two-channel CRD is expected to measure the PER of thin film PBSs with an uncertainty of ∼1.0% across the entire range from approximately 100:1 to >10⁹:1.